1. Field of the Invention
The present invention relates to a plastic fly eye lens used in an optical system such as a liquid crystal projector.
2. Related Background Art
As conventional fly eye lenses, fly eye lenses B and C, employed in a pair in a housing A of an optical system such as a liquid crystal projector are known, as shown in FIG. 5. These fly eye lenses B and C respectively have pluralities of cells E.sub.B and E.sub.C, and light from a light source can uniformly irradiate a target object with the optical function of the cells E.sub.B and E.sub.C.
The fly eye lenses B and C are fitted in the housing A, fixed with retainers H extending through the housing A with their outer edge portions abutting against projections F projecting on the inner surface of the housing A, and are positioned such that their lens surfaces perpendicularly intersect an optical axis Z of this optical system. In this state, when a lamp D emits light, this light is focused in units of cells E.sub.B of the fly eye lens B. The focused light is then received and enlarged in units of cells E.sub.C of the fly eye lens C that correspond to the cells E.sub.B in one-to-one correspondence. The light thus irradiates a target optical system G as uniform light.
In this conventional optical system, the interior of the optical system is heated to a high temperature of 150.degree. C. or more by irradiation with the lamp D. The housing A and fly eye lenses B and C thus thermally expand, so desired optical performance cannot be obtained.
For example, when the coefficient of thermal expansion of the housing A is larger than that of the fly eye lenses B and C, the housing A becomes larger with respect to the fly eye lenses B and C. Then, a gap is formed between the inner wall of the housing A and the outer edges of the fly eye lenses B and C, and the centers of the fly eye lenses B and C are shifted from the optical axis Z. In this case, the centers of the two fly eye lenses B and C are also shifted from each other, and the optical performance may undesirably, largely degrade.
When the coefficient of thermal expansion of the housing A is smaller than that of the fly eye lenses B and C, the fly eye lenses B and C become larger with respect to the housing A. Thus, the fly eye lenses B and C are to expand by pushing the inner wall of the housing A. Therefore, the fly eye lenses B and C may be distorted, leading to degrading optical performance.
As a countermeasure for this inconvenience, a glass plate I coated with an anti-reflecting coating may be interposed as a heat-insulating member between the lamp D and fly eye lens B, as shown in FIG. 6. However, as heat of the lamp D is conducted through the housing A and the like, thermal expansion of the housing A and fly eye lenses B and C cannot be prevented effectively.